The present invention concerns the use of liquefied gases and compositions of liquefied gases, for domestic use. For this domestic application, a gas mixture composition must be provided that is capable of burning under the fuel mix conditions required by the burner either of a boiler or of a domestic cooker, for the uninformed user.
Since the quantity of fossil gases is on the decline and there is a risk of long-term shortage thereof, there is a need to find compounds capable of substituting for these fossil gases. However, the development of new sources of gaseous fuels, and hence of substitution compositions for fossil liquefied gases, requires that these sources do not bring a major change in the behaviour of liquefied gas compositions and that these compositions can still be directly used in existing installations. Therefore these new compositions must be stable when stored, like existing compositions, and above all must be capable of burning continuously, for domestic use of gas sold in bottles and intended to be used with conventional burners or any type of burner, to supply heating installations.
International application WO 99/00466 describes a substitute fuel for internal combustion engines consisting of a mixture of several miscible fuels, of which at least one has a high octane number and at least one has a high cetane number. One of these substitution fuels consists of a liquefied mix of at least 2 gases, one being self-igniting by compression, the other being a good oxidizer. According to one embodiment, this fuel consists of a mixture of liquefied petroleum gas (LPG) i.e. a mixture of hydrocarbons comprising no more than 4 carbon atoms, and dimethylether (DME) with high cetane number. The use of said fuel can limit particulate emissions through incomplete combustion and pollutant emissions, compared with conventional fuels.
European patent EP 928,326 describes the use, in a dry low NOx combustion system, of a fuel composition consisting of a mixture of three components: 15 to 93 weight % dimethylether, 7 to 85 weight % of at least one alcohol and no more than 50 weight % of a component chosen from the group consisting of water and C1 to C6 alkanes. According to one particular embodiment, the third component is chosen from among water, methane, propane and liquefied petroleum gas. This combustion chamber is intended to supply a turbine for power production.
U.S. Pat. No. 6,202,601 describes a method to inject two gaseous fuels into a cylinder of a combustion engine, one being the primary fuel and the other being the pilot fuel which self ignites more readily than the primary fuel. The primary fuel is chosen from among natural gas, liquefied petroleum gas, biogas, landfill gas and hydrogen gas, and the pilot fuel is dimethylether. According to one particular embodiment of the method, the primary fuel and the pilot fuel are added separately to the engine, the primary fuel during a first stage, the pilot fuel during a second stage, these stages corresponding to different load operating modes of the engine.
Japanese patent application JP 60086195 describes a gaseous fuel composition obtained by incorporating dimethylether (DME) in a mixture of alkane and alkylene hydrocarbons containing 3 to 4 carbon atoms, to the proportion of 5-30 parts by weight DME in 100 parts by weight hydrocarbons: this mixture has good storage stability and good combustion properties. According to one particular embodiment, the 2 components, after being respectively liquefied, are mixed together before being added to the gas bottle. The gaseous composition comprises propane, n-butane or i-butane, but preferably propane alone with dimethylether or diethylether. This latter mixture notably has improved storage stability and improved combustion characteristics. It is even specified that this mixture is uniform and is able to burn continuously without interruption, and it therefore lends itself to use as fuel for industries or domestic applications.
It has now been found, and is the subject of the present invention, that liquefied gases containing dimethylether or DME in a mixture with at least one hydrocarbon compound with 3 carbon atoms and at least one hydrocarbon compound with 4 carbon atoms can improve storage stability whilst maintaining combustion properties close to those of a liquefied LPG gas. Contrary to the disclosure in Japanese patent JP 60086195, DME cannot be used alone with propane since the propane is consumed first, and when it is consumed DME becomes the majority component and the flame goes out. When used alone with butane, DME is the first to vaporize, and ignition and flame stability give rise to problems. In addition, to obtain regular combustion, it is preferable not to exceed the maximum concentration of 40% DME in the gas so that combustion in air is full and regular. However, if burners which can adjust the quantity of air could be provided for domestic use, optimization of parameters could allow more DME to be incorporated, up to 59 or 60% in the evaporated phase.
The subject of the present invention is therefore the use of a composition of storable liquefied gases containing dimethylether or DME in a mixture with a hydrocarbon mixture containing at least one hydrocarbon with 3 carbon atoms or propane and at least one hydrocarbon with 4 carbon atoms or butane, in which the gaseous mixture released after storage contains a constant DME concentration, fixed at a value of no more than 50 weight DME, until more than 50 weight % of the stored composition has been released. Therefore, said compositions can substitute for fossil liquefied gases currently available on the market for domestic use, in all types of existing burners, or even for use in an engine with combustion remaining constant and stable.
Preferably, in the use of the invention, the DME content in the released gaseous mixture is fixed at a value varying from 5 to 30 weight %. Preferably, in the use of the invention, the DME concentration is kept constant until more than 60 weight % of the stored gaseous mixture has been released. Preferably, in the use of the invention the initial gaseous mixture comprises 10 to 30 weight % DME, 20 to 50% propane and 50 to 20% butane. Preferably, in the use of the invention, the composition of liquefied gases has a DME/butane ratio of no more than 7 in the initial liquid mixture. Preferably, in the use of the invention, the composition of liquefied gases has a DME/propane ratio of no more than 6 in the initial liquid mixture.
Preferably, in the use of the invention, the composition of liquefied gases has DME/propane and DME/butane ratios which vary from 0.5 to 2. Preferably, in the use of the invention, the initial liquid mixture comprises 5 to 40 weight % DME, 5 to 91% propane and 90 to 4% butane. Preferably, in the use of the invention, the composition of liquefied gases is obtained from liquefied gases of commercial DME, butane, propane type. Preferably, the use according to the invention of the composition of liquefied gases is intended for the supply of domestic appliances, boilers, cookers and heating appliances in particular which operate by combustion in the presence of air, irrespective of the burner used. Preferably, the composition of liquefied gases is stored in a pressurized enclosure, of bottle or tank type.
By hydrocarbons with 3 carbon atoms is meant normal propane and isopropane, hereafter propane. By hydrocarbons with 4 carbon atoms is meant normal butane and isobutane, hereafter butane. By maintaining the DME concentration constant in the gaseous mixture on release from storage, in a preferred embodiment of the invention, is meant the ensuring of this concentration until more than 60 weight % of the gaseous mixture has been consumed.
The function of propane is to ensure the start of combustion and to ensure its stability, in general propane preferably burning before the DME and butane. The function of butane, burning after the DME, is to ensure continuous evaporation of the DME until its complete elimination from the storage, the butane ensuring maintaining of the flame. Evidently, the liquefied gas of the invention may contain other components such as the sub-products of DME synthesis, which are water and methanol. Therefore by “commercial” DME is meant a product containing at least 95% DME. By “commercial” propane is meant a mixture of hydrocarbons to the approximate proportion of 90% propane and propene, and the remainder being ethane, ethylene, butanes and butenes. By “commercial” butane is meant a mixture of hydrocarbons chiefly consisting of butanes and butenes and containing less than 19% by volume of propane and propene.
To obtain a constant DME concentration in the gaseous mixture, and to maintain good combustion stability and good heating value of the whole, it is necessary to adjust the DME/propane and DME/butane ratios in the gaseous mixture i.e. mixtures of liquefied gases in the storage capacity. Preferably DME/butane ratios of no more than 7 and DME/propane ratios of no more than 6 are chosen in the initial liquid mixture. Preferably, the DME/butane and DME/propane ratios vary between 0.5 and 2. In one preferred embodiment of the invention, the gas mixture placed in storage comprises 5 to 40 weight % DME, 5 to 91 weight % propane and 4 to 90 weight % butane. Preferably, it comprises 10 to 30 weight % DME, 20 to 50 weight % propane and 50 to 20 weight % butane.
One advantage of the liquid compositions of liquefied gases used according to the invention is their storage stability. Measurements taken on different proportions of mixtures over a period of 52 days showed no composition change. The storage of the liquefied gases used according to the invention can use conventional methods such as bottles, cylinders, containers, reservoirs or tanks intended for the storage of pressurized gases and/or liquids. Filling is carried out following usual methods. According to one preferred embodiment, the liquefied gases and compositions described above are used to supply a domestic appliance, in particular boilers, cookers and heating appliances operating by combustion in the presence of air, irrespective of the burner used. It can also be contemplated to use the above-described liquefied gas and liquefied gas compositions for the supply of internal combustion engines, notably of vehicles using on-board stored liquefied gas.